Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory

["Andy Lutomirski" <luto@kernel.org>]

On Tue, Aug 31, 2021, at 12:07 PM, David Hildenbrand wrote:
> On 28.08.21 00:18, Sean Christopherson wrote:
> > On Thu, Aug 26, 2021, David Hildenbrand wrote:
> >> You'll end up with a VMA that corresponds to the whole file in a single
> >> process only, and that cannot vanish, not even in parts.
> > 
> > How would userspace tell the kernel to free parts of memory that it doesn't want
> > assigned to the guest, e.g. to free memory that the guest has converted to
> > not-private?
> 
> I'd guess one possibility could be fallocate(FALLOC_FL_PUNCH_HOLE).
> 
> Questions are: when would it actually be allowed to perform such a 
> destructive operation? Do we have to protect from that? How would KVM 
> protect from user space replacing private pages by shared pages in any 
> of the models we discuss?
> 

What do you mean?  If userspace maliciously replaces a shared page by a private page, then the guest crashes.

(The actual meaning here is a bit different on SNP-ES vs TDX.  In SNP-ES, a given GPA can be shared, private, or nonexistent.  A guest accesses it with a special bit set in the guest page tables to indicate whether it expects shared or private, and the CPU will produce an appropriate error if the bit doesn't match the page.  In TDX, there is actually an entirely separate shared vs private address space, and, in theory, a given "GPA" can exist as shared and as private at once.  The full guest n-bit GPA plus the shared/private bit is logically an N+1 bit address, and it's possible to map all of it at once, half shared, and half private.  In practice, the defined guest->host APIs don't really support that usage.